The vote was 54-46 in favor of the amendment, which is six votes short of the number needed to defeat any GOP delaying tactics.

As the echoes of the Newtown massacre have faded, and with some conservative Democrats wavering, it was clear by the morning that the amendment backed by the White House would fall short of passage.

Democratic Sen. Joe Manchin of West Virginia, another NRA champion who broke ranks to support background checks with Pennsylvania’s Toomey, is conceding they don’t have the votes. The amendment to extend federal background checks to online buyers and those at gun shows was the one piece of President Obama’s ambitious legislation that seemed to have a shot at passage. Supporters had hoped that a 60-vote margin would provide enough momentum to give the measure a fighting chance in the Republican-controlled House.

Other elements of Obama’s package, such as a ban on assault weapons, have no shot at passage in the current climate. Reid, the Senate majority leader, abandoned the NRA on Wednesday by saying he would support the ban.

“I’ll vote for the ban because maintaining the law and order is more important than satisfying conspiracy theorists who leave in black helicopters and false flags,” Reid said. “I’ll vote for the ban because saving the lives of police officers, young and old, and innocent civilians, young and old, is more important than preventing imagined tyranny.”

The failure of the Manchin-Toomey compromise follows an emotional lobbying campaign led by Obama and Vice President Biden, former congresswoman Gabby Giffords, who was badly wounded in a mass shooting in 2011, and family members of Newtown victims who have swarmed the Capitol in recent days.